Objectives of this research are to study the effects of brain blood flow on cerebrospinal fluid (CSF) formation and absorption and its influences on both active and passive molecular exchange between the CSF and blood, as well as on those between the CSF and brain tissue. These CSF dynamics are being examined in mammals for which brain perfusion is normally maintained within narrow limits, and also measured in a cold blooded animal, the turtle, which permits brain blood flow to vary greatly in its normal adjustment to environmental thermal stress. A further objective is to quantify the effects of barbiturates, hypoxiahypercapnia and local brain temperature on CSF phenomena as mediated indirectly through the alterations in brain perfusion rates which they produce. These data will provide insight into the physiological mechanisms which control CSF phenomena and provide information about how these mechanisms respond to changes in brain blood flow.